Field of the Invention
This invention relates generally to sample injectors for analytical instruments such as gas chromatographs and, more particularly, to an arrangement for mounting and shielding the septum from exposure to the interior of the sample injector. In certain analytical instruments such as gas chromatographs, it is necessary to introduce a sometimes minute quantity of a sample to be analyzed by injecting it from a sample probe, customarily a hypodermic syringe, into a flowing stream of gas. The gas, carrying the injected sample, then passes through the chromatographic separation column and the effluent gas passed to a detector all in a manner well known in the art. A septum, most commonly a membrane of a physically permeable self-sealing material such as silicone rubber seals the entrance port of the injector block. The sample is introduced by piercing the septum with the injection probe, the puncture sealing itself when the probe is withdrawn.
It has long been recognized that it is desirable to interpose between the inner surface of the septum and the interior of the injector block a shield of inert material in order to prevent what is known as "septum bleed". This term refers to the tendency of the septum material, albeit relatively inert, to outgas or exhaust volatile components which are either constituents of the septum material itself or comprise analytical samples absorbed during a prior use of the injector.
With the development in recent years of improved detectors of extremely high sensitivity, the problem of septum bleed has become increasingly troublesome and a variety of remedies have been employed to cope with the problem. The basic approach common to these remedies is the use of a septum shield, which, in its simplest form as shown in U.S. Pat. No. 3,581,573 (FIG. 2) is a disc of inert material which overlies the face of the septum exposed to the interior of the injector block. The probe needle, when inserted, pierces both the septum and the shield; when withdrawn only that relatively small area of the septum which is in registration with the puncture hole in the septum is exposed to the interior of the injector block.
Another form of septum shield, shown in the aforementioned patent as well as in U.S. Pat. No. 3,635,093, employs a strip of shielding material disposed in sliding contact with the inner face of the septum. The ends of the strip are accessible from the exterior of the injection block so that, after the probe has been inserted and withdrawn, the puncture made in the shielding material can be moved out of registration with the septum which is then completely shielded from the interior of the injector block.
Both of these approaches to the problem of septum bleed have shortcomings: in one instance, the former, complete shielding of the septum is not achieved; and in the latter, it has proven difficult if not impossible in practice to achieve a satisfactory seal with the movable strip of shield material. In this connection it should be borne in mind that the high temperatures and moderate pressures encountered greatly complicate the sealing problem.
Another disadvantage is that, in all forms of septum shields shown in the above-referenced patents, it is necessary to remove the shielding element in order to replace the septum which, of course, must be done quite frequently. And in some forms, where the shield is a disc of probe-penetrable material, it is necessary or at least good practice to replace the shield as well as the septum. If this is not done, the puncture hole in the shield becomes enlarged or the shield is punctured in different places and the end result in both cases is that a larger area of the septum is exposed to the interior of the injector.
It is the primary general object of the present invention to overcome or at least mitigate the shortcomings of prior art devices as pointed out above.
A specific object is the provision of an improved septum shield arrangement in which the septum is totally shielded from the interior of the injection block and in which the septum can be replaced without removing the shielding member.
A further object is the provision of an improved septum mounting and shielding arrangement in which it is unnecessary to replace the shield and which is tightly sealed against leakage.